What phenomenon causes the "brown cloud" seen in Denver during winter?

The phenomenon known as the "brown cloud" commonly observed in Denver during winter is primarily caused by increased pollution from vehicles. During winter months, temperature inversions often occur in the area, trapping colder air near the surface, while warmer air resides above it. This stagnant air prevents pollutants, such as soot and other particulate matter from vehicles, from dispersing. As a result, the concentration of these pollutants increases, leading to the formation of a brownish haze that's visible over the city.

Increased humidity, high levels of precipitation, and increased air temperature at higher altitudes do not directly contribute to the formation of the brown cloud. While humidity can affect visibility and air quality, it is the pollutants that primarily cause the brown color. Precipitation can help clear out particulate matter, and air temperature variations mostly influence whether or not inversions occur, but the root cause of the brown cloud is tied to pollution levels. Thus, the most accurate understanding relates to the high levels of emissions from vehicles during colder weather conditions leading to an accumulation of pollutants in the atmosphere.